Have you tried just tightening the chain tension? Loosen the axle nuts, pull the rear wheel tight against the chain, and re-tighten the nuts, just tight enough so the chain doesn't bind when you turn the cranks. Use a box end wrench. An adjustable will just round off the corners. You'll still have to re-tighten from time to time. Getting it right will also take some practice. The nature of the beast.

If you really need a new chain, I can only tell you that mountain bike chains are made for derailleur-equipped multi-speed cogs that are too narrow for single-speed cogs. Without seeing your current chain or measuring the width of the cogs I wouldn't want to make a recommendation